Iron and cobalt are the central metal ions in many complexes that participate in oxidation or reduction catalysis in living systems and in various commercial applications. The heme proteins are examples. In this research, attention will be focused on the ways in which the oxidation-reduction properties of iron and cobalt respond to carefully controlled variations in the structures of macrocyclic tetradentate ligands to which they are bound. Pertinent properties include standard electrode potentials, reversibility of electrode reactions, availability and stability of extreme oxidation states and ability of the compound to function in the appropriate reaction: e.g., decompose hydrogen peroxide, oxidize a substrate, form a complex with molecular oxygen. Broad ranges of metal-containing enzymes catalyze nucleophilic processes and it is often assumed that the active site brings several factors to bear in catalysis (polyfunctional catalysis). Two modes of metal ion activation are well established for the hydrolysis of the esters and amides of amino acids and of peptides by relatively uncomplicated metal complexes. In both cases, the metal ion is recognized as performing in two ways to promote the process. Special structures are being developed in which a metal complex may contribute in three and, perhaps even four, ways to the activation of hydrolysis processes. This is an approach toward polyfunctional homogeneous catalysis.